1. Field of the Invention
The present invention relates to an optical instrument equipped with a zoom lens such as a photographic camera, an electronic still camera, a video camera, an overhead projector, a regular projector and the like.
2. Description of Related Art
Typically, various optical instruments such as cameras are equipped with zoom lenses. Such a zoom lens has two principle functions, namely a zooming function of varying a focal length to change the magnification of an image and a focusing function of focusing the zoom lens on an aimed object to form a sharp image. These two functions are achieved by causing relative movement of a plurality of lens elements of the zoom lens partly or wholly along the optical axis of the zoom lens.
There have been known various zoom lens drive systems (a)-(d) for achieving the two functions.
(a) Two motor system in which two electric motors are used to drive a focusing mechanism and a zooming mechanism, separately; PA1 (b) One motor system in which a single electric motor is used to drive selectively a focusing mechanism and a zooming mechanism; PA1 (c) A step zoom system adapted to cause zooming at a plurality of fixed zoom ratios in which a single electric motor is used to alternately drive a focusing mechanism and a zooming mechanism while the motor rotates in one direction such as known from, for example, Japanese Patent Publication No. 6(1994)-100707, and PA1 (d) System that has been developed and installed in cameras on the market in the name of, "Epion 210 Zoom" and "Epion 270 Zoom" by the applicant of this application. In this unique system, there is provided rotational play between lens barrels of the zoom lens in which a plurality of lens groups are installed, respectively. For example, one of the lens barrels rotates following another lens barrel which is driven in one direction by an electric motor during zooming-up operation, and remains stationary until the other lens barrel is driven in the other direction through a rotational angle equivalent to the rotational play and rotates following the other lens barrel after the rotation of the other lens barrel through the rotational angle equivalent to the rotational play during zooming-down operation after the zooming-up operation. The zoom lens is adjusted in focus by means of relative rotation between the two lens barrels.
The first two zoom lens drive systems (a) and (b) need a number of constructional parts, which is always disadvantageous in terms of size and cost reduction. The third zoom lens drive system has to have lens barrels having large angles of rotation for an increased number of fixed zoom positions, which results in providing a complex driving mechanism and increasing the overall size of the zoom lens and making the camera bulky. The zoom lens drive system (d) is simple in mechanism and needs only a small number of constructional parts. However, there is a disadvantage that stops positions of the one lens barrel are inaccurate in the extent of the rotational play due to no members for restraining the one lens barrel in the fixed zoom positions and, in consequence, the one lens barrel is possibly displaced in position due to and impact, which leads to an image out of focus.